The subject matter of this disclosure relates to a new repair method for hot parts of turbomachines based on the technique known as hot isostatic pressing (HIP). Hot isostatic pressing is a manufacturing process, typically used to reduce the porosity and internal defects of metals in order to improve the material's mechanical properties and the yield of the production process.
The HIP process submits the component to be treated, placed in a high pressure vessel, to both elevated temperature and elevated pressure, applied to the component in isostatic manner, through the use of a pressurizing gas. Inert gases are most widely used so that the material to be treated cannot chemically react during the process.
The HIP process is typically used for fixing or reducing shrinkage cavities and others internal casting defects. The simultaneous application of heat and high pressure works to eliminate or reduce material flaws like internal voids and micro-porosity through a combination of plastic deformation, creep, and diffusion bonding, thus improving mechanical resistance of the part treated and in particular its fatigue.
Furthermore, HIP can be used for the consolidation of powders, and for diffusion bonding—the solid state joining of two or more parts—(e.g. metal cladding or bonding of material not weld-able by fusion processes—the bonding together of dissimilar metals).
HIP is widely used to fix structural defects on metal components obtained through investment casting process and powder metallurgy process. The components are subject to a number of possible internal defects like, but not limited to: gas porosity, shrinkage cavities, cracks, hot tears, internal lack of fusion or bonding, and metallurgical defects. After the investment casting or the powder metallurgy process is completed, the resulting metal component is carefully examined to identify possible defects. Internal defects are normally fixed by HIP process. Alternative processes to fix mentioned defects are welding and brazing. This alternative processes are widely employed to repair or restore static parts, whereas their application is limited for rotary parts, being particularly critical parts. These methods typically show an important drawback: the component needs to be cleaned up through machining and mechanical treatments that can be expensive, difficult to perform and time consuming. The high pressure and temperature driven by HIP is capable to fix only embedded discontinuities (cracks, porosities, cavities, etc.)